1887 | Metschnikoff on Germ-Layers. 335 
arise, gives origin to a peripheral layer of cells, slightly differing 
from the rest which they include; and this layer subsequently 
divides into two,—the inner, lying in contact with the included 
yelk, being called the mucous layer, and the outer, exposed to 
surrounding agencies, being called the serous layer: or, in the 
terms used by Professor Huxley in describing the development 
of the Hydrozoa, the endoderm and ectoderm. This primary 
division marks out a fundamental contrast of parts in the future 
organism.” The share each layer takes in building up the devel- 
oping animal is then touched upon, after which the author pro- 
ceeds to draw an analogy between the layers of an animal body 
and the grades of society, in which he compares the ectoderm 
with the higher grades, the endoderm with the lower grades, and 
the mesoderm with “ers-étaz. 
Huxley’s theory for a long time found no supporters in Ger- 
many. In that country there was noticeable a certain reaction 
against the general application of the doctrine of the germinal 
layers. This reaction reached its greatest height in the well- 
known paper by Weismann on the embryology of the Diptera. 
- Such a tendency was in perfect accord with the doctrine of types 
then prevalent, according to which morphological comparisons 
could only be made within the limits of one and the same great 
oup. ; 
The similarities in the structure and development of animals 
were long regarded as the expression of a universal plan, which 
was of a purely ideal nature. In the last two decades it has 
been generally recognized that at the bottom of these similarities 
lies genetic relationship. The value of embryology as a key to 
this relationship was recognized by Darwin, who laid special 
emphasis on the view that the embryo, being less differentiated 
‘than the adult, ought to afford us valuable information concern- 
ing the structure of its ancestors; and that when the embryos 
of two animals are alike, the similarity is due to a common 
descent. He attempted to illustrate these general laws by con- 
crete examples, and where he met exceptions, he explained them 
by supposing the embryonic record to be obscured by larval 
adaptations and precocious inheritance. Darwin thus came to 
the conclusion that the parallel Agassiz had pointed out is due 
to the blood relationship of animals, and that this parallel is 
thoroughly revealed only in cases where the process of develop- 
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